The present invention relates to a push-pull output circuit in an audio output amplifier, or more in particular to a Class B push-pull output amplifier like a Class A amplifier which is not cut off.
Generally, the output stage of an audio output amplifier uses a push-pull output circuit of the Class A or B complementary type in view of the recent trend toward larger output and higher performance. The Class B push-pull output circuit has a higher efficiency than the Class A push-pull output circuit and therefore is used in most output power amplifiers. In spite of this advantage, the Class B push-pull output circuit has the disadvantage that a switching distortion is caused by repetition of on-off operation of output transistors in negative and positive phases of a signal. Specifically, in the Class B push-pull output circuit, the two output transistors in push-pull connection are prevented from producing a crossover distortion by applying a forward bias voltage to the two output transistors in such a manner that a slight amount of current flows simultaneously in the two output transsistors in the absence of a signal. In the case where the input signal voltage applied to the input terminal of the output transistor is of such a polarity as to reversely bias the output transistor, however, the output transistor is cut off when the input signal voltage gradually increases to a level exceeding the forward bias voltage applied to the output transistor.
Thus the Class B push-pull output power amplifier circuit is such that the output transistors are turned on and off alternately every positive and negative half cycles of the input signal. With the increase in signal frequency, the time delay for transferring from on to off state or from off to on state becomes considerably long, thus causing a switching distortion. Since this switching distortion contains very high harmonics, the improvement by negative feedback is substantially offset. For this reason, the only method of reducing the switching distortion is to use a high speed element capable of high-speed on-off switching operation. Even if a MOS FET which is the highest speed element available at the present is used, however, it is impossible to completely eliminate the time delay, and the switching distortion is not essentially avoided in Class B operation.
The Class A operation in which the output transistor is not cut off through the time range of the input signal and therefore the switching distortion is not caused, is low in efficiency, thus making it practically unsuitable for a large output amplifier.
If a circuit is configured not to cut off the output transistor over the entire time range of the input signal like the Class A circuit while at the same time retaining the feature of the Class B circuit high in power efficiency, it is possible to obtain a push-pull amplifier high in efficiency which does not cause any switching distortion. Such an amplifier is disclosed, for instance, in U.S. Pat. No. 3,995,228, as the basic circuit thereof is shown in FIG. 1. In this circuit, two output transistors Q.sub.1 and Q.sub.2 of emitter follower type and drive transistors Q.sub.5 and Q.sub.6 for driving the transistors Q.sub.1 and Q.sub.2 make up a Class B push-pull circuit. The voltage V.sub.BE representing the total sum of the base-emitter voltages V.sub.BE1, V.sub.BE2 of two output transistors Q.sub.1, Q.sub.2 and the base-emitter voltages V.sub.BE5, V.sub.BE6 of two drive transistors Q.sub.5, Q.sub.6 is generated by two V.sub.BE multiplier transistors Q.sub.3 and Q.sub.4. Thus a crossover distortion is prevented from being generated in such a manner that the output transistors Q.sub.1, Q.sub.2 and the drive transistors Q.sub.5, Q.sub.6 are conducted by the generated V.sub.BE voltage in the absence of an incoming signal applied to an input terminal V.sub.IN. In order to produce a quiescent bias voltage supplied to the four transistors Q.sub.1, Q.sub.2, Q.sub.5 and Q.sub.6, the current flowing in a resistor R.sub.2 due to the base-emitter voltages V.sub.BE3 and V.sub.BE4 of the two transistors Q.sub.3 and Q.sub.4 which are applied across the resistor R.sub.2 is supplied to two other resistors R.sub.1 and R.sub.3, thereby attaining a voltage drop across each of the three resistors R.sub.1, R.sub.2 and R.sub.3.
The operation in response to an incoming signal will be described. Assuming that a positive input signal is applied to the input terminal V.sub.IN, the emitter current of the output transistor Q.sub.1 increases which in turn increases the voltage drop across an emitter resistor R.sub.E1. Also, the base-emitter voltage of the output transistor Q.sub.1 slightly increases, thus increasing the voltage between nodes A and Z. In view of the fact that voltage at a node X is clamped to the voltage at the node Z by a constant-voltage source V.sub.X, namely, the voltage at the node X is fixed at a voltage which is the sum of an output voltage V.sub.OUT of this circuit and the voltage of the constant voltage source V.sub.X, the current flowing through the resistor R.sub.1 increases thereby increasing the voltage drop across the resistor R.sub.1, thus increasing the bias voltage between the node A and a node B. The voltage at a node Y is clamped to the voltage at the node Z by a constant-voltage source V.sub.Y, and therefore does not change with the increase in the voltage between the nodes A and Z, so that the voltage between the nodes B and Z does not change. As a result, a quiescent current flows in the transistors Q.sub.2 and Q.sub.6 during the absence of an input signal and therefore these transistors are not cut off. Thus, upon application of a negative signal voltage to the input terminal V.sub.IN, it is immediately amplified thereby to prevent a switching distortion from being caused. This circuit is required to change the bias voltage by use of an input signal current, and therefore has the disadvantage that it is impossible to produce a satisfactory large output. Specifically, when the voltage between the nodes A and Z increases by the input signal voltage, the current flowing in the resistor R.sub.1 is required to be increased in order to increase the voltage drop across the resistor R.sub.1. As this increment current, the input signal current is used. Thus, the input signal used for amplification decreases by the value flowing in the resistor R.sub.1 thereby making it impossible to produce a large output. The attenuation of the input signal means the reduction in the open loop gain of the amplifier, thus leading to the shortcomings of a decreased negative feedback and increased distortion for a closed loop.